NBS TECHNICAL NOTE 816

Engineering

and Construction Manual

for an Instrument to Make

Burn Hazard Measurements

In Consumer Products
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the -United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

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THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing addiess Washington, DC. 20234.

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3 Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Engineering and Construction Manual for an

Instrument to Make Burn Hazard Measurements

in Consumer Products

National Bureau of Standards

R 2 9 1974

. °. °

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Louis A. Marzetta

Measurement Engineering Division

Prepared for Consumer Product Safety Commission

Washington, D.C. 20016

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U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued February 1974
National Bureau of Standards Technical Note 816

Nat. Bur. Stand. (U.S.), Tech. Note 816, 48 pages (Feb. 1974)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:816). Price 85 cents.
Table of Contents

Page

Abstract 1

1. Introduction 1

2. Design and Construction Details of the Probe 1 2.1 Fabrication of the Probe with Ribbon-Type Thermocouple Element . 2 2.2 Fabrication of the Probe with a Fine-Wire Thermocouple Element . 5

3. Circuit Description 6

4. Placing the Instrument Into Operation 8

5. Testing Methods 9 5.1 Test with Glass Reference Surface 10

5.2 Effect of Thermocouple Depth Variation 11

5.3 Test with Plastic Coating on Metal Surface 12

5.4 Additional Comments on One Second Measurements 12

5.5 Summary of Comments on the Topic of Contact Time and its

Relationship to the Physiological Data of Fig. 19 13

6. Influence of Variation in Thermal Inertia ' 14 6.1 Influence of Variation in Controlled Probe Temperature 14

7. Summary of Errors . 14

8. Additional Comments and Conclusions 15

APPENDIX A and B 16

References 17

Table 1 List of Components and Materials 18

Table 2 Values of erf(y) 21

List of Figures - 22

Engineering and Construction Manual For an Instrument

to Make Burn Hazard Measurements in Consumer Products

Louis A. Marzetta

Surface temperature measurement alone is insufficient to establish

the hazard of human contact with a hot or cold object. A metal surface

is more likely to cause thermal injury than a plastic surface at the

same temperature. An instrument equipped with a measuring probe has

been developed for indicating the tissue temperature that would be

experienced if human contact were made with the hot surface in question.

The correct value of interface contact temperature can be read for a selected contact time without knowing the composition or temperature of

the heated material under test.

A detailed set of instructions and drawings for assisting in the

construction of the device is included in the manual. Several test

procedures are described for use in checkinq the performance of the

measuring probe and instrument.

Key words: Consumer products; contact temperature; thermesthesiometer

;

thermometry.

1. Introduction

When assessing the burn hazards of solid surfaces it is a fact that a metal

surface is more likely to cause thermal injury than a plastic surface at the same temperature Ell 1

. An instrument has been developed for measuring this effective burn

hazard. The device is able to duplicate the tissue temperature that would be

experienced if human contact were made to a hot surface regardless of its composition.

The measured contact temperature can be translated to a qualitative evaluation of the

hazard for the heated surface in question.

A principle from heat flow theory was selected as the basis for the instrument

design. It can be predicted analytically that a steady-state interface temperature

exists when two masses at different but constant temperatures are brought into contact.

Laboratory tests confirmed the possibility of measuring this interface temperature.

Since the purpose of the instrument system is to make a realistic contact measurement,

the measuring probe was designed to simulate human tissue. By selecting a material for

the instrument probe with thermal properties close to that of a human finger and

temperature regulating the probe to the value for human tissue, the instrument reading

accurately duplicates the temperature of an actual finger contact. A short analysis of

the system development is currently being prepared for publication [2] . The

instrument is called a thermesthesiometer—a device for measuring the human sensibility

to heat.

2. Design and Construction Details of the Probe

The construction of the probe is shown in a simplified schematic cross section in

Fig. 1. A heater wire is wound around the inner core tube and a resistance thermometer

transducer in physical form similar to a strain gage element is positioned along the

central axis of the core tube. These two components form the basis of the temperaturecontrolling circuit that maintains the probe assembly at a finger-tissue temperature of

33°C. The interface temperature-measuring thermocouple element is positioned about

0.01 cm beneath the outer surface of the probe face, while its electrical companion

junction is located about 1.5 cm back in the temperature-controlled core region. All

the components are encapsulated in the silicone-rubber compound. The probe is mounted

in a holder for a comfortable hand clasp and is provided with a wiring cable and

connector.

1 Figures in brackets indicate the literature references on page 17.

1
2.1 Fabrication of the Probe with a Ribbon-Type Thermocouple Element

The assembly process of the internal probe elements begins by mounting of the

front thermocouple; this step is followed by winding the temperature controlling heater

coil; next, the 100 ohm resistance thermometer is positioned along the central axis and

the element lead wires are soldered to a terminal block that has been mounted in place;

finally, the probe structure is encapsulated in its holder tube with the special

silicone-rubber medium, and the external cable and probe handle assembly are installed.

Figs. 2 through 7 are a series of drawings for fabricating the mechanical

components that make up the probe assembly, and in most part the drawings are selfexplanatory. To assist in visualizing the assembly process the sketch in Fig. 8 can be

referred to as each step is undertaken.

The assembled thermocouple ribbon element is not at present commercially available

from stock. It is necessary to obtain the two ribbon materials of Chromel and Alumel,

about 0.1 cm (0.040 inches) wide by approximately 0.0076 cm (0.003 inches) thick, (see

list of components and material). A thermocouple element is made from 2.5 cm (oneinch) strips of the two materials electrically spot welded with a junction overlap of

about 0.025 cm (0.010 inch).

STEP 1. Trim the thermocouple element to an overall length of 4.8 cm (1-7/8 inch),

2.4 cm (15/16 inch) each side of the welded junction). Prepare a pair of 7.6 cm (3-inch) lengths of plastic insulated (red and blue) #30 tinned copper

wire with ends bared for about 0.6 cm (1/4 inch); insulated wire used for

wirewrap applications is suitable. Wrap the bare end of the red insulated

wire around the last 0.3 cm (1/8 inch) end of the Alumel ribbon (the one that

has magnetic attraction) , solder the connection using a stainless steel flux.

Attach the blue insulated wire and the Chromel ribbon in the same manner. Coat the soldered connections with ordinary liquid epoxy cement and allow to

dry.

2. Carefully bend the ribbon element around two sides of a 1.3 cm (1/2 inch)

square wooden dowel with 0.15 cm (1/16 inch) radius edges to form a hairpin

loop with a somewhat flattened top and slip the loop inside the probe core

tube Fig. 2, and position it as shown in the assembly sketch of Fig. 8. Center the loop between the mounting holes in the lip of the core tube. Fine

wire such as the #30 used in step 1 can be twisted through the holes in order

to hold the thermocouple loop in position; the loop should extend about 0.024 cm (3/32 inch) beyond the core tube end. At this point it is important to

insure that the thermocouple junction is precisely located uppermost at the

loop tip and that the ribbon surface is correctly plane-oriented. For this

step as well as others it is strongly suggested that an illuminated optical

magnifier or stereo microscope of 5 to 20 power be available for adjustment

and checking during the assembly process. Cement the element in place with

liquid epoxy; do not coat the junction area with cement. The twisted wires

holding the thermocouple can be clipped short and left in place.

3. Starting from the lower hole in the probe core body, the heater winding coil

is wound upward towards the upper hole. The winding starts with a protruding

length from the lower hole of about 3.8 cm (1-1/2 inches) beyond the bottom

end of the tube, and it should terminate at the upper hole with about the same length. The winding will consist of about 14 turns for a 74 cm (29-

inch) piece of the #36 enamel insulated nichrome wire. Sharpened toothpicks

may be used to hold the winding in place at the holes while liquid epoxy

cement is applied to the winding and during curing.

4. Trim the sides of the DN-100 resistance thermometer elenent slightly to

achieve a sliding fit inside the probe core tube. Slide the element in place

in a right angle plane to the mounted thermocouple ribbon. This will place

the element along the center of the tube with one of its edges alongside the

heater winding holes. The top of the resistance element is set flush with

the front face of the probe core tube; cement the element in place. Slip a 0.8 cm (5/16 inch) length of teflon spaghetti over each lead, positioned near

the element.

2
5. Bend the copper wire junctions at the ribbon thermocouple inward at an angle

of about 45 degrees as shown in Fig. 8. These connections form the reference

junction which is to be embedded in the temperature-controlled RTV material.

Extra care is required in encapsulating the probe assembly to insure that the

measuring thermocouple junction is precisely located at the depth equivalent

to the dermal layer in human skin [3] . This is necessary if the probe is to

accurately simulate the contact temperature response of a human finger. A

casting assembly fixture with a built-in micrometer is used for this purpose.

Fig. 6 is a construction drawing of the device and Fig. 7 is an assembly

diagram showing the probe components in relation to the casting fixture.

6. At this stage all three of the electrical elements have been mounted in plac

with their six wire leads protruding from the back end. These leads are to

be attached to a terminal block in the next step. A suitable terminal

junction block for the probe can be made by modifying a commercial component,

a 14-pin integrated circuit socket ordinarily used with a printed-circuit

board, such as Cinch - part no. 14-DIP-l. A pair of terminals is sawed away

from each end of the socket leaving 10 terminals. The four corners can be

rounded with a file, and the socket is fitted in place in the back cap

section of the probe core tube. Before cementing the block in place the red

and blue thermocouple lead wires are threaded through the center hole, the

resistance thermometer leads are each brought through an end hole; the heater

winding leads are threaded through the nearest opening along the side of the

terminal block. The lead notation for soldering to the terminals is given in

Fig. 9. A stainless steel flux is used with the heater winding after

scraping away the insulation with fine sandpaper. Wherever possible a slack

loop should be provided at each lead.

7. Turn the micrometer adjustment disc-screw in the casting fixture so that the

movable brass end plate rests about 0.3 cm (1/8 inch) below the ledge of the

aluminum body; this distance is to allow sufficient clearance for the 0.24 cm

(3/32 inch) protrusion of the thermocouple loop tip at the probe core tube.

8. Place the lucite probe body Fig. 4 (part C) in place in the casting fixture

with the internal ledge in the upright position. Make a mark on the lucit^

tube at its bottom exterior that coincides with a mark made on the aluminum

fixture body. Lock the lucite tube in position with the side thumb screw.

9. Slide the probe core tube assembly inside the lucite tube until it seats

firmly in position. The assembly can be held in place with a small

toolmaker's clamp. Make a positioning mark on the inner core tube that

coincides with a mark made at the inside of the lucite tube. This set of

marks and the set described in step 8 are for use in precisely repositioning

the components

.

10. Connect an ohmmeter between one of the thermocouple terminals and the locking

screw on the side of the casting fixture.

11. Turn the micro meter adjustment inward at the point where the ohmmeter just

indicates that the thermocouple junction has contacted the micrometer plate.

This represents zero equivalent skin depth. Back off the micrometer

adjustment by the desired equivalent depth; the micrometer screw has a travel

of 0.0635 cm (0.025 inches) per revolution. A setting of between 0.0076 and

0.0125 cm (0.003 and 0.005 inches) depth is suggested. Note the zero and

final adjustment points on the micrometer adjustment disc-screw by means of

appropriate markings.
12. Revert to Step 7 and repeat the procedure several times to insure that the

final value depth adjustment can be repeated to a precision of around 0.0025 cm (0.001 inches). Coat the inner and outer surfaces of the thermocouple

loop, including the junction area, with a coating of Eccosil Primer No. 33,

and let dry- The purpose of this coating is to insure the adhesion of the

silicone rubber material to the thermocouple.

In the search for a suitable material to be used in encapsulating the probe

assembly several factors were considered. The part of the probe that would be pressed

against the various heated surfaces should have some resiliency; it would have to

withstand high temperature exposure, as much as 200 "C. The material would need an initial flow property to allow a near perfect encapsulation of the intricate assembly.

Finally, the thermal properties of the substance should combine to yield a thermal

inertia close to that of human tissue in order to satisfy the "simulated" finger

analysis. A silicone-rubber material seemed a likely choice. Several companies offer

silicone products that have been doped with metal compounds in order to tailor their

thermal characteristics. One such material is ECCOSIL2 4952; it is a two part pourable

RTV silicone-rubber compound with the required thermal inertia.

13. The encapsulating process begins with the preparation of the RTV material.

The bulk portion of the material should be thoroughly mixed in its original

container by means of a stirrer rod in a drill press as well as hand stirring

with a heavy spoon. The substance has a thick consistency and a tendency to

settle out heavily at the bottom of the container.

14. Weigh out, 100 grams of the stirred bulk material in a paper cup on a platform

balance.

15. Add 0.5 grams of the liquid catalyst to the cup with the aid of a medicine

dropper while the cup is on the balance.

16. Stir mixture vigorously with a spoon.

17. Place the cup in a vacuum chamber and evacuate until a head of bubbles break

at the surface of the RTV material. Break the vacuum and stir the material

again.

18. With the aid of a disposable funnel made from a square piece of paper having

about a 0.6 cm (1/4 inch) spout opening, fill the probe lucite body that has

been mounted in place in the fixture described in Step 8 to a level of about

0.6 cm (1/4 inch) below the inner ledge of the lucite body.

19. Lower the inner probe assembly into the liquid material taking care to orient

it according to Step 9. Lower the assembly slowly and finally firmaly

against the shoulder stop; hold in place with a small toolmaker's clamp.

20

.

Place the fixture in the vacuum chamber and bring vacuum down to the point

where bubbles are breaking at the surface, but do not pump down further in

order to avoid excessive RTV material rising into upper chamber of the probe

core. Hold the vacuum for about 8 minutes, then gently release the vacuum. Allow the assembly to cure in the chamber in ambient air for a period of

about 24 hours.

Certain commercial components and materials are identified in this report in order to

specify the experimental procedure adequately. In no case does such identification

imply recommendation or endorsement by the National Bureau of Standards, nor does it

imply that the material or component identified is necessarily the best available for

f-h*=> the purpose. nnrnnco
21 . After the curing period the encapsulated probe can be removed from the

fixture and made ready for assembly to the handle and cable. A six-conductor

cable is cut to a rough length of 127 cm (50 inches) . One end of the cable

is threaded through the hole in the probe handle. Connections to the

terminal block in the probe can be made according to details in Fig. 9. A

cable clamp is secured above the terminal block connections for the purpose

of relieving any strain on the connections after the cable has been pulled in

place in the handle. The remaining end of the cable is attached to a male

panel connector as shown in Fig. 9. During this assembly a 0.9 cm (3/8 inch)

length of teflon spaghetti should be slipped over each soldered pin

connection. Finally, check the entire cable assembly with an ohmmeter for

continuity of correct end-to-end connection, a check of freedom from crossleakage, and the correct ohmic readings for each of the probe elements

according to values in Fig. 9. The probe handle is held in place as a result

of its machined snug fit in the remaining probe assembly; if looseness exists

it can be remedied with a coating of cement. Remove any raised edge at the

probes face resulting from the encapsulation process.

2.2 Fabrication of the Probe with a Fine Wire Type Thermocouple Element

To date there have been nine different models of probe constructed. The details

presented here represent the latest version of a suggested design. Evaluation tests

are being continued with the device; its experimental nature implies that future

modifications in design can be expected. It is also likely that variations of probe

geometry will be considered in order to adapt the instrument to new and varied forms of

thermal measurement applications.

The previous description of probe construction was centered around the use of a fabricated ribbon-type thermocouple. If there is difficulty in obtaining chromel and

alumel material in ribbon form or if there are problems in making the welded junction,

it is possible to use the commonly available round-wire thermocouples. However, it is

the author's opinion that the ribbon-type thermocouple may be a better configuration

for the probe. Bare chromel-alumel thermocouples, 0.00 76 cm (3 mil) diameter wire, can

be readily obtained with a round ball, welded tip. A probe can be constructed using

this type of thermocouple but it will be necessary to modify the first five step

instructions, as follows:

Step 1 (modified) The ribbon type thermocouple is physically replaced by a strip of

vinyl material about 4.1 cm (1-5/8 inch) long, approximately 0.0076 cm (0.003

inches) thick. Bend the vinyl strip into a flattened hairpin-like loop as described in the previous Step 2, and slip the loop inside the probe core

tube while centering the loop between the mounting holes in the lip of the core tube. As described for the ribbon thermocouple, the loop can be held in

place with fine wire; the loop should extend beyond the core tube end by

about 0.24 cm (3/32 inch). Cement the strip in place with the two-part

liquid epoxy adhesive.

2 (modified) Locate and cement the thermocouple bead junction on the outside

centerline of the strip, and very precisely set the bead in its upright

position at the topmost point of the strip arc. An optical magnifier or stereo microscope is necessary during this procedure. The base leads are brought down the outside surface of the plastic strip, with a genereous

separation, to the rim of the core tube end as shown in Fig. 10, and are cemented down the inside of the core tube.

3 (unchanged) Starting from the lower hole in the probe core body, the heater

winding coil is wound upward towards the upper hole. The winding starts with
a protruding length from the lower hole of about 3.8 cm (1-1/2 inches) beyond

the bottom end of the tube, and it should terminate at the upper hole with

about the same length. The winding will consist of about 14 turns for a 74 cm (29 inch) piece of the #36 insulated nichrome wire. Sharpened toothpicks

may be used to hold the winding in place at the holes while liquid epoxy

cement is applied to the winding and is subsequently drying.

4 (modified) Trim the sides of the resistance thermometer element with

scissors to achieve a sliding fit inside the probe core tube. Remove the

element and make two pairs of mounting holes in the bottom area of the

element for use as junction points for thermocouple ends and the copper lead

wires as shown in Fig. 10. Loop the copper lead wires through their mounting

holes at the resistance element, and slide the element into the probe core

tube, part-way along the center of the tube, with one of its edges along the

inside of the heater winding holes. At this point the bare thermocouple

leads are trimmed to a suitable length and are attached to the copper wire

junctions. The alumel (magnetic) wire is soldered to the red insulated lead

using the stainless steel flux and the chromel wire to the blue lead. Slip

the resistance element in place in the tube to where it is flush with the end

face of the core tube and is at right angles to the thermocouple strip loop.

Carefully check to see that the bare thermocouple wires are properly routed

down the inside of the tube before looping over to the end of the resistance

element; the wires must not touch each other. Cement the element in place as well as any loose thermocouple wires. Coat the thermocouple copper junctions

with cement, and slip a 0.8 cm (5/16 inch) length of spaghetti over each of

the resistance element leads.

5 (delete)

.

6 through 21 Same as previous description.

3. Circuit Description

The circuit shown in Fig. 11 is the complete electronic diagram of the instrument.

The circuit can be broken down into two major sections an analog section is used for

amplification and temperature control, and the digital circuitry is concerned with

timing and output indication. Integrated circuit components are used for both the

analog and digital sections. The digital panel meter is a standard, compact 3-1/2

digit package with a full-scale sensitivity of 19 9.9 mV. A standard 5V, 1A supply

module powers the digital circuitry and the panel meter, while a 5V to ±15V converter

module is used with the operational amplifier circuitry.

A 100-ohm resistance thermometer located within the probe is used in a bridge

circuit at the input of amplifier Al for the temperature regulation of the probe at 33°

C. Power amplifier Ql supplies the probe heater current. In the event the probe

absorbs too much heat from prolonged contact with a hot surface, the temperature

controller will indicate this condition by an indicator lamp actuated by transistor Q2

and the imbalance signal from Al. It is necessary to equip the instrument with an aluminum plate at room temperature in order to avoid excess heating of the probe.

After each reading the probe is brought into contact with the heat sink for a few

seconds to remove the excess heat. Amplifier A2 is highly stable and amplifies the

chromel-alumel thermocouple output. Chrome 1-alumel material was chosen for its high

linearity of output voltage versus temperature and for its rugged physical properties.

The function of the digital timing circuitry is to allow the selection of a precise contact measuring time with the following timing sequence. Within a few

milliseconds after firm contact between the probe face and hot surface under
measurement, the digital panel meter logic circuitry delivers to the pulse generator

circuit Ul, an initiating positive voltage step corresponding to the 40°C crossing of

the meter reading. The actual timing takes place at U2 upon receiving a pulse from Ul.

A front-panel switch allows the selection of contact reading times of 1, 2, 4, or 8 seconds. At the end of the timing period, JK flip-flop stage U3 supplies a gate pulse

to the digital panel meter via Q3, which causes the last temperature reading at the

meter to be "frozen" for ease of reading. Panel indicators alert the operator to the

functioning of the timing sequence, and a push-button is used to reset the meter before

each new reading.

There are no scale-factor calibration controls in the instrument. Initially the

electronic circuit is adjusted for a controlled temperature of 33 °C at the probe by means of a bias adjustment at the input of stage Al, using an independent thermometer

for measuring the probe temperature. After the controlled temperature of the probe has

stabilized, a second control located at the input of stage A2 is adjusted for a reading

of 33 degrees at the digital panel meter.

The majority of the electronic components are located on a double sided printed

circuit board; the components include the analog and digital active integrated circuits

and their associated resistors and capacitors. The +5 volt to +15 volt dc to dc

converter module is located on the board, and a male connector plug mounted on the card

for the external leads. The board also includes the adjusting pots for the probe

temperature control and the 33 °C digital panel reading.

Fig. 12 and Fig. 13 are layouts of the two sides of the printed circuit board. A

list of component values is given in Table 1. All components including the dc to dc

converter, are mounted on the side of the board shown in Fig. 14.

The printed circuit board is fabricated by the standard photo-etched process using

copper-clad fibre-glass board material. The resultant printed circuit was plated for ease in component soldering. Since the project was an engineering development effort,

the active components were made to plug into socket terminals mounted on the board for ease of component exchange; however, new users may wish to solder the active componentsdirectly in place. In the case of the IC operational amplifiers Al, and A2, spring

sockets were soldered into the board, AMP Corp. part no. 50462-8, using a no. 55 drill

for the board holes. Walden/Molex IC Soldercon Terminals no. 05-30-001 were used on

the board for the digital logic modules Ul, U2, and U3; a no. 70 drill was used for the

board holes in this case. The board connector was made up from AMP post pins, part no. 85931-6 using a no. 63 drill for the board holes, a push fit for the pin shank tab.

Care should be taken when soldering these pins into the board to insure that the solder

does not wick up the sides of the pin, otherwise poor physical mating with the cable

connector will result. The cable connector is AMP part no. AMPMODU 1-86256-7 with

AMPODU contacts no. 86015-2 attached to cable leads by means of the appropriate AMP

crimping tool. The remaining components in the board are soldered in place in holes

made with a no. 60 drill. All drilling is done from back side of the board, Fig. 13.

It is very important to be aware of the numerous feed-through connections that

have to be made between the board surfaces. In most cases the feed- through connection

is made by the leads of a resistor or capacitor. Note that there are printed

connections on both sides of the board at some of the IC terminals; take care in

soldering not to allow any molten solder to flow into the openings of the IC socket

receptacles.
4. Placing The Instrument Into Operation

It is assumed that the instrument has been properly wired according to the wiring

diagram in Fig. 11. Correct value fuses have to be inserted in their locations; one

fuse in the primary power line of the master five volt power supply and the other one

in the five volt line that feeds the digital panel meter. Two operational amplifier IC

modules and three digital IC modules have to be plugged into their locations at the

printed-circuit board; take care to orient the digital IC modules according to the

information shown in Fig. 14. The following are suggested steps for setting up the

initial adjustments to the system.

STEP 1. Before turning on power set the two trimming potentiometers located at the

bottom side of the printed-circuit board to their maximum ccw position,

approximately twenty complete turns.

2. Connect the probe to main instrument by means of the panel connector; connect

the power cable to the rear of the instrument; turn the power switch to ON

position; push reset button panel.

3. Adjust the trimming resistor pot. R17 in a cw direction until the panel meter

reads 33°. 0. This control does not affect the temperature control of the

probe; rather it is the system temperature datum level. However, the

selected value of 33.0°C corresponds to the controlled probe temperature

which in turn relates to the specified human tissue temperature of 33.0°C.

Allow the device to warm up for 30 minutes and re-adjust R17 for a 33.0

reading if necessary.

4. Adjust trimming resistor pot. R5 in a cw direction until the overheat LED

panel indicator extinguishes. The control sets the controlled temperature of

the probe. What is wanted is a control temperature of 33.0°C at the internal

section of the probe. Since this measurement is difficult, an external

temperature check has been found to be effective. The working surface of the

probe is monitored with a fine-wire thermocouple used in conjunction with a temperature-indicating instrument. Adjustment R5 is brought up cw in 1/4-

turn increments with about a five-minute wait between settings until the

probe face temperature is measured to be between 31 and 32° C. These values

correspond to the values obtained experimentally on the surface of human

skin. The panel meter of the instrument will show a reading of about 32

degrees if the probe is left resting and untouched at its face for a room temperature near 25° C.

An additional simple check on the operation of the instrument can be made at this

time. If the probe face is brought into contact with a slab of brass or aluminum that

has been resting at room temperature, the instrument will show a contact temperature

reading that is one or two degrees above the room temperature for a contact time of

about 10 seconds. If the room temperature material that is contacted happens to be

non-metallic such as wood or plastic the instrument reading will be significantly

higher, perhaps only a degree or so below the 32 degree quiescent level. This

corroborates the sensation of coldness when human contact is made with metal objects at

environmental temperatures.

The following suggestions are based on experience to date. The manner of holding

the measuring probe described here is suitable for most working areas; however, there

may be situations with surfaces oriented in ways that require a different grasp of the

probe. For normal measurements the lucite probe body is held with the thumb on the

front side facing the user and the middle and forefinger on the opposite side. The

other two fingers are tucked into the palm away from contact with the hot surface.
The greatest expertise to be mastered is the initial contacting of the heated

surface with the probe face. This author found that the best results were obtained by

using a wrist motion with some snap action. The objective is to bring the probe face

up parallel to the surface in a quick movement, resulting in a firm, square and

positive contact. This dexterity is necessary to insure precision measurements,

especially at the one-second contact time. After a short period of experience, the

operator can expect a repeatability of measured temperature to within two degrees for more than 90% of the trials. In the case of four-and eight- second contact time

measurements, the application of the probe to the working surface is not as critical as with the shorter time tests.

As indicated elsewhere in this report the degree of contact pressure that is

applied does not noticeably affect the readings; a 4 to 8 newtons (one- or two- pound

force) is suggested. After noting the panel reading the Reset button can be pressed to

cancel the reading in preparation for the next test. The usual meter reading before

each measurement with the probe not in contact with any surface will be between 30 and

34 degrees. The operator should remember to apply the probe to the room- temperature

aluminum plate between readings.

Experience obtained in the construction of several models of the instrument shows

that the accuracy of the 1,2,4, and 8 second timing is assured if the stated values and

tolerances of components: R29, R30, R31, R32 and C3 are adhered to. However the

timing should be checked. This can be done by bringing the probe in contact with a heated metal surface of about 60 to 70° C. Each of the selected contact times can be

checked by measuring the interval between the activation of the Time and Read panel

lights with the aid of a stop watch. The agreement at one second contact time should

be within 0.15 seconds and within 10% of the other selected values.

Temperature Limitation of Probe: The maximum temperature reading of the

instrument is 199.9° C (392 "F) . While the silicone rubber probe material is

commercially rated to withstand an exposure of about 255 °C (490*F), it is suggested

that the probe not be subjected to contact times greater than 4 seconds for

temperatures between 125 and 200 °C. This is to insure that the thermal properties of

the material are not affected, otherwise the accuracy of the instrument may be degraded

There is some evidence that a repeated exposure of the silicone rubber material to

temperatures above 100 °C causes a gradual hardening of the substance. At no time

should the probe be allowed to be subjected to a temperature above 275 °C (527 *F) , even

for a brief instant; this will permanently damage the probe.

5. Testing Methods

In the previous discussion in Section 4 the two circuit adjustments were concerned

with the initial settings for the controlled probe temperature and for its quiescent

reading. There are no scale factor or calibration controls. The measuring system was designed on the principle of a calculable transducer whose transfer function was described previously. The signal amplification is fixed by means of precision value

resistors R22 and R19 , at stage A2 in Fig. 11, so that a correct normalized value of

contact temperature will be read by the digital panel meter for a probe constructed

from a chromel-alumel sensing thermocouple. The amplification has been normalized at

24.3 to correspond with the chromel-alumel thermal emf of 41.2/^//°C, between 33 and 63°

C. This results in a transfer function of lmV/°C at the digital panel meter. The

validity of the readings from an instrument designed along the stated principles can be

substantiated by comparing the experimental test values of temperature with those

values obtained by calculations from theory. This test procedure will be discussed

next.
5.1 Test With A Glass Reference Surface

This test ascertains if the instrument reading is a true measure of the

contact temperature as predicted from heat flow theory. A glass material is a suitable

working surface and it can be used as a heated reference standard. In addition to its

stable qualities, glass has a thermal inertia that is desirably located between high

thermal conductivity metals and low conductivity plastics. A suggested standard

reference glass is Corning type 7940 fused-silica. It is a stock item available in a number of physical sizes; a convenient one is about 5 cm (two inch) disc with a thickness of approximately 1.25 cm (one-half inch). The optical grade with a no. 2 finish is suitable for the application. A substitute material is a flat piece of

window glass about one cm thick. With a knowledge of the values of the various thermal

parameters it is possible to calculate from heat flow theory the contact temperature at

a given surface temperature of the heated glass material. This temperature can be

compared with the instrument reading obtained by contact of the instrument probe and

the heated- glass surface.

The procedure is to heat the glass material on the surface of a hot plate. (A

controlled temperature water-bath technique can be used.) An adjustable transformer is

used to set the input electrical power at various levels. The temperature of the

reference glass surface is measured with a fine wire thermocouple (0.0075 cm or less)

in conjunction with a digital indicating thermometer having one-tenth degree

resolution. The thermocouple bead tip can be pressed down on the glass surface by the use of a very low density material such as a small piece of silicone foam rubber; in

the case of surface temperatures below 85° C a small piece of packing material-styrofoam

plastic-may be used. A commercial surface- thermocouple, sensing probe may be used in

place of the fine wire element for ease of handling; one such device is the Model

68004-K surface probe made by the Omega Engineering Co. However its greater mass can

be expected to contribute a greater offset reading error than a fine wire thermocouple.

For a surface temperature of 80° C the fine wire element may read one or two degrees low

whereas the commercial sensing probe may have an offset of four or five degrees at this

temperature. The measurement of surface temperature T

h

, is probably the principal

source of error in the testing procedure.

The object of the experiment is to vary the temperature of the glass surface in a step-wise fashion over a range from about 50 to 100° C. At each step the stable surface

temperature T^ is recorded and the contact temperature T c is measured by the probe and

instrument. The instrument contact time is set to four seconds, which means that the

reading is essentially a steady state value of T c

. Figure 15 is a plot of the

instrument values of T c at each measured value of T^ in relation to a locus line

representing the values of T c as calculated from heat flow theory and described in

Appendix A. All measured points are included in the figure for this representative

test run. If an instrument and probe have been constructed according to the stated

principles, the comparison results for this particular test should show an agreement

between the measured T c and the calculated value to within plus or minus three degrees over the indicated range.

The expectancy of measurements within ± three degrees of the correct value of T, is a statement of accuracy. The observed operational precision shows a repeatability

of measurement where more than 90% of the readings can be expected to have a total error of ±2.0 degrees or less. This is a test of repeating the T measurement with the

probe and insuring that the glass T^ temperature remains invariant from each testing

point to the next one. This is also a good test to demonstrate the fact that the

applied pressure between the probe face and the heated surface is not critical; the

force applied can be varied over a large range but a force of 4 to 8 newtons (one or

two pounds ( has been found to be both satisfactory and comfortable. This insensitivity

to contact pressure and to a deliberate introduction of a water or oil film to the

probe face (though not recommended) indicates the contact resistance interface between

the probe and the heated mass can be neglected.

10
5.2 Effect of Thermocouple Depth Variation

The previous test was a series of temperature readings of T c for a fixed contact

time of four seconds. This means that in the region of the measuring thermocouple at

the probe face there has been sufficient time to reach temperature equilibrium, and

there is practically no temperature difference between the probe surface and a plane at

the thermocouple depth. While the previous test was a valid check on the instrument's

ability to measure a quasi steady-state contact temperature, the test did not yield

sufficient information to establish a complete correspondence to human contact. If the

thermocouple element had not been encapsulated at the correct depth corresponding to

the dermal layer in human skin, the previous test will not reveal the defect. The

following test procedure offers a means of determining the equivalent skin depth

location of the encapsulated thermocouple element at the probe face; in addition the

test data presents information to check the short-contact-time (one second) response of

the probe. Also the profile of the plotted contact temperature as a function of time

when applied to a heated surface of unknown composition can offer clues as to its

thermal behavior.

Using expressions derived from heat flow theory and presented in Appendix B, a series of curves was calculated using four different values of depth measured from the

probe face to the thermocouple junction corresponding to different depths of the dermal

layer in human skin tissue. The results are shown in Fig. 16. The abscissa is the

contact time and the ordinate values are the temperature at the various depths.

The analysis procedure is the plotting of experimental probe data on the figure

and comparing its profile with the existing curves. The broken line function in Fig.

16 is plotted from values measured with the probe. Contact temperature measurements, TJ, were made with the heated glass reference held precisely at a constant temperature T n , before each of the 1,2,4, and 8 second contact readings. The exact value selected for

the heated surface is not important (between 80 and 9CP C is suggested) ; the same temperature must be maintained for each of the time tests. The four values of contact

temperature are plotted on the figure; for ease of comparison an arbitrary fixed value

of abscissa shift was added to each temperature in order to place the curve conveniently between a pair of the existing functions in the figure. Of concern here

is the slope-profile of the plotted probe data. In the example in the figure the slope

of the test curve appears to be almost equivalent to the no. 2 function, perhaps a bit

less. This means that the measuring thermocouple junction was encapsulated below the

probe face at between one and two equivalent "epidermal skin" depths. This seems to be

typical for probes built to date. If the plotted test curve had shown a slope closer

to the no. 4 function, it could be suspected that the probe sensing element was set too

deep and would not be acceptable. For such a probe the contact temperature values for

8 seconds contact time would be reasonable but the one-second readings would be much

too low as indicated in the figure. Note that the ordinate values in this figure have

been physically expanded in order to accentuate the slope differences.

If a probe has been found to have a thermocouple junction set too deep it may be

possible to remedy the situation. Some of the RTV material can be carefully removed

from the probe face by gently rubbing it against a fine grade of silicon carbide paper,

(about 600 grade); the face should be checked with a straight edge. (It is important

in these tests that the probe face be flat and be free from a raised edge produced

during encapsulation of Section 2.1) . The described procedure can be used to monitor

the degree of material removal by observing the improvement in the slope at the one and

two second region, but in no instance should the material removal expose the

thermocouple element.

The argument presented here is that the sensing element depth was responsible for

the slope-contour of the probe response curve. While the sensing depth is the

principal factor, the probe response is also influenced by the small but unavoidable

thermal time constant of the thermocouple element. Probably this added factor accounts

for the typical probe having a response slope somewhat greater than expected even

11
though the thermocouple junction may have been set precisely to a single "skin" depth

at encapsulation. The small discrepancies in sensing depth can he neglected since the error involved is within the assigned accuracy of the instrument. Also, there is

debate concerning the correct skin depth since the dermal layer varies between

individuals, as well as from one part of the body to another.

5.3 Test With Plastic Coating on Metal Surface

The previous tests show that a probe tested in the manner described and found to

have its sensing element correctly set should not imply that all future measurement

data could not show an anomalous behaviour as indicated by an extra-large slope in the

time-response curve. The test was conducted on the fused-silica reference surface,

which is a nearly ideal semi-infinite-length thermal mass. There will be heated

surfaces encountered in practice that are far from ideal whose contact temperature as a function of contact time is impracticable to predict from theory. Indeed, it is the

contact time flexibility of the instrument that allows a correct reading to be made

with these thermally-complex cases. Fig. 17 shows the results of a -test on a heated

surface altered by a plastic coating. A thick piece of brass was maintained at a constant T, temperature, and a series of contact-temperature readings at the contact

times were made and plotted in curve A. A portion of the brass surface was covered

with a piece of 0.013 cm (0.005 inch) teflon tape, and the tests were repeated on this

surface. The second test data is plotted in curve B, and it is obvious that there is a much larger slope to curve B, especially in the 1 and 2 second contact-time area. Though the plastic film is thin, its lower thermal conductivity has influenced the

transport of heat. This example has potential practical significance; it suggests that

the makers of consumer products can use the instrument as a design tool to meet safety

specifications. The simple expedient of covering a hot surface with a thin coating of

plastic can significantly reduce the contact temperature at ' all contact times and

especially so for short contact times. This latter point is important since the

situation of brief contact with hot surfaces is very dangerous and is of increasing

concern.

5.4 Additional Comments on One Second Measurements

The electronic details of the instrument and some features of the probe could be

simplified by designing the measuring system for a fixed contact-measuring time of , say, four or eight seconds. By reducing the circuit complexity, the fixed longer

measuring time would have reduced depth positioning tolerance of the thermocouple

element in the probe to some degree since the thermal gradient from probe face to

sensing element becomes quite small at contact times above four seconds. With a single

measurement with the longer contact time it would be possible to predict the one-second

contact time temperature by an extrapolation of the data using scale factors taken from curve 1 or 2 in Fig. 16. For the majority of applications the results are valid, but

it is possible that a heated surface such as the one described in conjunction with Fig.

17, might be encountered. It is possible with non-ideal thermal masses to have

anomalous readings with one second contact. There are significant operational

advantages with the short measuring periods. A larger number of points at a surface

can be covered than with a longer measuring time. Also the repeated measurements at

the longer periods require a correspondingly long time interval between readings in

order to remove the excess heat from the probe with the aid of the passive plate. The

routine manner for using the instrument will be to make a series of quick measurements

of a heated surface with the instrument set to a one-second contact time. Whenever a suspicious temperature has been read the procedure will be to make a series of readings

at the location at all the contact times. It may be desirable to make a simple graph

plot of this data for the record and subsequent study. Accuracy of the data is likely

to be better for a four-second measurement than at one second, the smaller contact time

demanding a greater dexterity of the operator.

12
The one-second contact measurement time has an additional relationship with the

operation of the instrument. The ordinate valiies in the physiological curves of Fig.

18 are for steady-state surface contact temperatures rather than dermal temperatures;

this conforms with the related data found in the literature. The temperature

difference between surface and dermal skin regions is most noticeable at the short

contact times. Because this difference is a function of contact time, the dermal

temperature is not constant throughout the coi.iact period. Hence the indicated

temperature is slightly higher than the physiologically effective value. Conversely

the thermal response lag of the thermocouple element causes the instrument reading to

be on the low side. The two errors taken individually are small, but since they have

an opposing effect, the net error is almost negligible. Any error is undoubtedly

submerged in the uncertainty of the physiological data with one second contact, since

this data was extrapolated from tests on skin where the shortest direct observation

time of tissue damage was for a five-second application of heat.

5.5 Summary of Comments on the Topic of Contact Time

and its Relationship to the Physiological Data of Fig. 19.

Currently it is impossible to state how safety regulations will be worded with

respect to limits of contact temperature and contact time. The interest in a onesecond contact stems from the commonly experienced momentary touch with a hot surface

followed by the reflex action of fast withdrawal. It has been found that the pain

sensation and reaction is established in about 0.4 to 1.0 seconds. The one-second

contact time is also unique because the threshold-of-pain temperature coincides with

the skin-damage temperature in this time area. This is in contrast to the wide

temperature separation between initial pain and skin damage for the four-second contact

region as shown in the physiological curves of Fig. 18. [4] The upper solid-time

curve in Fig. 18 is the temperature at which epidermal destruction can be expected, as

a function of contact time. The curve immediately below the one described is the

threshold of epidermal injury that may be reversible. The lower dotted curve is the

threshold of pain sensation.

A safe value of dermal contact temperature for a four second measurement is

unconditionally safe for a one second contact. However, a safe measurement value at one second is not necessarily safe at four second contact. One reason for this

statement stems from the physiological response curve which shows that the hazardous

contact temperature is about 10 degrees higher at one second contact than it is at four

seconds. In addition, due to the dermal-depth gradient function the measured value of

contact temperature is lower at one second than at four seconds.

Clearly it would be easier to meet the safety standards for a one second contact

than for the four second time. Also, (see section 5.3), simply coating a hot surface

with a thin plastic can result in a dramatic decrease of contact temperature,

particularly for one second contact. This procedure could lead to a dangerous

situation. For example, a steel surface can be covered with a plastic coating less

than 0.015 cm (0.006 inch) thickness. For a surface temperature of 102C C, the contact

temperature on the plastic coated steel is 60 degrees for a one second contact - a value that could be rated safe. Yet the four second contact temperature on the same surface would be 73 degrees - a value that is well into the hazardous region.

13
6. Influence of Variation In Thermal Inertia

An exact value for the thermal inertia of the RTV material in the probe is not

required. Likewise, small variations in the controlled probe temperature 1L will not

significantly affect the measurement accuracy of the instrument. Expression (1) in

Appendix A shows that these factors, in virtually all cases, have less than a one-toone effect on the value of T

c

. For example an analysis of (1) was conducted allowing

for an exaggerated variation in the ratio of A*, /J\f an<3 determining the resultant

variation of Tq for several different heated materials. The results are shown in Fig.

19 for a minus 50% uncertainty in the correct value of thermal inertia Xf . The graph

shows that a glass surface at 70° C would have its contact temperature read 6% high or

3.5 degrees (Tc of glass is about 50° C for the stated Th ) . In the case of a heated

metal surface at the same surface temperature the error would be about 1.5 degrees.

Since the uncertainty in the thermal inertia of the probe material is likely to be less

than 20%, the net error may be neglected.

6.1 Influence of Variation in Controlled Probe Temperature

The effect

can be explored.

of a variation in the controlled probe temperature on the T c reading

Fig. 20 shows the results of calculations of the error at for

various heated materials using an exaggerated 27% in the controlled probe temperature.

As in the preceding example, the error is quite small for metal surfaces and increases

for materials with low thermal inertias.

Fig. 20 is based on va

in Appendix A. In practice

also affect the reference j

variation in reference junc

Fig. 20. Hence the net

and it could be a negative

a probe with an external r environment - over 33°C.

lues calculated from the heat flow formula of expression (1) a variation in the internal probe temperature T D

, would

unction of the measuring thermocouple. The error flue to the

tion temperature is opposite in effect to the error shown in error in practice would be less than shown in many materials

percentage error for metals. It may be advisable to design

eference junction for anticipated work in a high temperature

7.0 Summary of Errors

The following is a summary of the possible instrument errors. Since the

magnitudes of the first two errors listed are a function of the heated surface's

thermal properties, the stated values are for the heated glass standard. The errors would be less for heated metal surfaces.

Error in T c due to a 20% deviation from assumed value of thermal

inertia for the RTV material used in the probe. 3.2%

Error in T c due to a 15% variation in the controlled probe -face

temperature T . 2.5%

Error tolerance for chromel-alumel thermocouple wire at 60 °C. 1.2%

Linearity error for chromel-alumel thermocouple plus

electronic amplification uncertainty. 0.25%

14
For uncorrelated errors the rms Net error =

\J (3.2)

2 + (2.5)

2 + (i. 2 )

2 + (0.25)

2 = 3.65%

For the standard reference glass surface at 90 °C and a contact temperature of 60 °C, the

T error is ±2.2° C. Over a period of six months involving many hundreds of

measurements with eight different probes, the observed errors have been within the

stated figure in most tests. This is the basis for rating the accuracy of the

instrument, as noted in section 5.1 to be about ±3 °C over the operating region of

interest - namely from 45 to 75 °C for T .

8. Additional Comments and Conclusions

Various other tests were conducted with heated reference materials ranging from

high conductivity metals such as copper down to insulating plastics and natural wood.

In almost all cases the comparison errors between measured and calculated values of Tc were within 3 degrees. A similar degree of conformity was observed in comparisons

between the instrument values and interface temperatures made with actual finger

contact on a number of surfaces. A few brief observations were taken with heated nonsolid surfaces such as heated air and water. In all situations, regardless of the

medium, if a particular environment caused a finger pain sensation for a 3 to 5 second

exposure time, the probe-instrument for the same exposure would read between 45 and 50° C as indicated on the human pain threshold line in Fig. 18.

This was especially evident in the case of blowing hot air at 65° C from a hair

dryer and near still, hot air at 250°C in an oven; both resulting in nearly the same physical sensation and instrument reading for about the same exposure time. One last

observation involved contact with cold surfaces rather than heated ones. Since the same theoretical aspects apply and because the electronic system and digital indicator

can function in a reverse direction, the overall system might be used to measure human

sensibility and exposure or contact hazard in frigid environment applications. Fig. 21

is a photograph of the packaged experimental instrument and probe.

The project was initiated by the former Bureau of Product Safety of the Department

of Health, Education and Welfare; the bureau was recently transferred to the Consumer

Product Safety Commission. The author wishes to thank Dr. Y. C. Wu of NBS for making

available his study material and numerous references on the subject of the hazards in

human contact with heated surfaces. The assistance of other members of the staff

deserve credits: K. F. Plitt, for his valuable suggestions in the choice of materials;

H. D. Kratz, for his efforts in modeling the mechanical desiqns of the instruments'

probe; P. J. Tobin, for the numerous drawings that are included in this report and to Rosemary Massengill for her untiring typing work in the preparation and restructuring

of the manuscript.

15
APPENDIX A

Derivation of Function in Fig. 15, referred to in Section 5.1

The interface or contact temperature between two masses brought into contact can

be predicted by a formula derived from fundamental heat-flow theory. The contact

temperature T , measured by the instrument can be stated as [5]

L-Tk - -r,

0) i+ vik/2,

where T^ is the temperature of the heated surface contacted by the probe whose own

initial temperature is Tp;^ and,}- are the thermal inertias of the heated mass and the

probe respectively. Various types of glass can be used for,\ h

; a suggested standard

reference is Corning type 7940 fused-silica. The data sheet supplied with the glass

has extensive information concerning the thermal properties of the material, including

the specific heat, thermal conductivity and density. The thermal inertia *\ , the

product of these three factors, can be readily calculated. The units given are in

calories, centimeters, grams, seconds and degrees Celsius; these units are convenient

since they are the ones used most in both the literature and in commercial product data

sheets (1 calorie equals 4.18 joules, approximately.) At 50°C, Corning 7940 glass has

a specific heat c, of 0.18 cal/gm°C;oit has a thermal conductivity k, of 3.3 x 10"^

cal/s cm °C; the density f , is 2.2 gm/cm . The calculated thermal inertia^ is 1.3 x

10 cal /s cm *C 2";^= kc p . The thermal inertia dif fusivity o<, , is important in the

calculation of temperature at various skin depths with contact time. For Corning the

calculated diffusivity is , , / > , / ^l+o

With regards to the RTV silicone rubber compound in the probe, its thermal inertia

X i was calculated from the data sheet information supplied with the Eccosil 4952

material and found to be 1.7 x 10~^ cal/s cm °C *". This value lies ideally between the

extremes of the values for thermal inertia of human skin... The most accepted documented

values of A for human skin range from about 1.2 x 10

-

-^ to 2.2 x 10" . The value of

thermal inertia for the RTV probe material may vary from one batch to another by

perhaps 10%; however the accuracy of measurement by the instrument does not suffer in

direct proportion. Expression (1) shows that the thermal inertia factors do not have a one-to-one relationship with the value of measured T

c .- Section 6.

APPENDIX B

Derivation of Functions in Fig. 16 referred to in Section 5.2

The internal temperature distribution in a solid at an initial temperature Tp

, can

be calculated from the following expression which is a solution to the Fourier "heat16
flow equations assuming one-dimensional flow. The temperature T^, at depth x from the

interface and at time t from the instant of contact is given by 1.5 J T - O)

where ^Xp is the thermal diffusivity of the probe or finger in contact with the source of

heat and

is the error function, a tabulated integral; a convenient list of values is given in

Table 2. If the diffusivity is given in cm /s as is the usual case in material data

sheets, than x is in centimeters and t is in seconds. This corresponds to the

dimensions shown in Appendix A. Expression (2) is useful in establishing the

temperature in the dermal region of human skin for a stated contact temperature,

contact time and penetration depth. Of immediate interest is the use of the formula

for determining the temperature gradient in the probe. A series of curves were generated using four different values of depth measured from the probe face to the

thermocouple junction and corresponding to different depths of the dermal layer in

human skin. The results are plotted in Fig. 16.

References

1. Wu, Y.C., "Material Properties Criteria for Thermal Safety",

Journal of Materials, JMLSA, Vol. 7, No. 4, December 1972, pp 573-579,

2. Marzetta, L. A., "A Thermesthesiometer - An Instrument for Burn

Hazard Measurement", to be published in outside journal.

3. Stoll, A. M. in Advances in Heat Transfer, Vol. 4, J. P. Hartnell

and T. F. Irvine, Jr., Eds. Academic Press, New York, 1967.

4. Henriques, F. C. and Moritz, A. R. , American Journal of Pathology,

Vol. 23, 1947, pp. 531 and 695.

5. Carslaw, II. S. and Jaeger, J. C, Conduction of Heat in Solids,

2nd ed. Clarendon Press, Oxford, 1959.

17
Table 1

List of Components and Materials

Resistors

All resistors are 1/4 watt, 2% tolerance, tin-oxide film type except where

noted; all values stated in ohms.

Rl - - 10k

R2 - - 10f)k

R3 -- 560k

R4 -- 510

R5 -— 2k potentiometer, Beckman model 63WR2k

R6 -- 91k

R7 -- 150k

R8 -- 150k

R9 -- Ilk

RIO -- 5.1k

Rll — 5.1k

R12 — 100

R13 -- Ik

R14 ~ 1.5k

R15 — 240k

R16 -- 240k

R17 — 2k po tentiometer, Beckman model 63WR2k

R18 — 100

R19 — 100k selected to 0.1% accuracy

R20 ~ 5.1k

R21 — 5.1k

R22 — 3.9k selected to 0.1% accuracy

R23 — 1.8M 5% tol.

R24 — Ik

R25 « 390

R26 — Ik

R27 — Ik

R28 — 680

R29 — 200k "X

R30 — 360k \ ON SW2

R31 — 680k 5% tol. >

R3 2 — 1.3M 5% tol. /

Capacitors

CI — l^F, 50V, Erie "red-cap" ceramic

C2 — 22 pF Mica

C3 — 4.75>t<F solid tantalum (timing capacitor) spec. no. CS13B-F475K

C4 — 330 pF mica

C5 — .Q1/U.F ceramic

C6 — . 01X,F ceramic

C7 — .01 UF ceramic
Semiconductor Components

CR1 ~ 1N3 064 diode

CR2 — 1N3064 diode

CR3 — 1N4729 zener diode 3.6V

CR4 — 1N4729 zener diode 3.6V

CR5 — LED, Fairchild type FLV-100

CR6 — LED, Fairchild type FLV-100

CR7 — LED, Fairchild type FLV-100

Ul — IC, 74121, one-shot multivibrator

U2 — IC, Signetics NE555V, timer, dual inline package

U3 — IC, 74107, J-K Flip-Flop

Al — IC, operational amplifier National Semiconductor, model LH0042CH

A2 — IC, operational amplifier , Analog Devices Model AD504K

Ql — transistor, 2N5191

Q2 — transistor, 2N3638, PNP General Purpose

Q3 — transistor, 2N364 3, NPM General Purpose

Switches and Connectors

SW1 — miniature power switch

SW2 — miniature rotary switch, Alco type MRA-2-5S

SW3 — miniature push button switch, normally open

PI — AMP Corp. AMPMODU pin posts, part no. 85931-6

P2 — AMP Corp. AMPMODU cable connector, no. 1-86256-7 and contact no. 86015-2

plus crimping tool no. 90202-2

P3 — probe cable plug, Amphenol no. 126-19 5 P4 — panel socket, Amphenol no. 126-19 8 P5 — power line filter socket and line cord, Corcom EMI filter no. 185,

line cord no. 80-1255

19
Additional Components and Materials

Digital Panel Meter, 0-100.0 mV, California Instruments type 83330, San Diego,

California - also a second source is Analog Devices, model AD2010, physical

size is slightly different from previous unit. The digital panel meter

must have the following specifications: A full scale range of o to 199.9

millivolts dc; BCD output line, positive true logic, for initiating the

timing sequence at the 40 millivolt crrossing; external hold line, logic

hold, for "freezing" the contact temperature reading; an internal triggered

conversion rate of at least 4 conversions per second.

PS1 — dc to dc converter module, 5V to 15V, Semiconductor Circuits Co., model

DD5-2.15.25, also L-TRON, type LPA, 5-2.15-30, L-TRON Co., Fairfax, Virginia

for a second source.

PS2 — power supply module, 5V, 1 ampere with unregulated 8 to 10 volt tap,

Semiconductor Circuits Co., type P.C. card no. 1.5, 1000, also L-TRON type

LT-1.5,1000 for a second source.

Printed Circuit board spring terminals for digital IC modules, Walden/Molex

IC Soldercon terminals no. 05-30-0001.

Printed Circuit board spring sockets for IC op amps, AMP Co., part no. 50462-8.

Resistance thermometer for probe, type DN-100, RDF Corp., Hudson, N.H.

Thermocouple ribbon wire, Chromel and Alumel, three inch strips about 0.040

inches wide and 0.003 inches thick; Namac Corp., Framingham, Mass.

Alternate - fine wire thermocouple, Chromel - Alumel, three mil diameter

wire with welded ball junction; RDF Corp. or Omega Engineering Corp.

Heater winding resistance wire with insulated coating, #36 gage, 0.005

inch dia., 31.7 ohms/foot, Driver Harris Co., Karma type insulation

or equivalent.

Six Conductor cable, Alpha Wire Co., no. 1176.

RTV Silicone Rubber compound for probe, ECCOSIL 4952 and ECCOSIL Primer No 33,

Emerson and Cuming Inc.

Standard Reference Surface: Fused Silica, Code 7940, Optical Grade no. 2,

circle - 2-1/2 inch dia., 1/2 inch thick, Corning Glass Co.

Instrument Cabinet, presently under consideration, possible use of cabinet

Wamco part no. 6153, special order, W. A. Miller Co., Oquossoc, Maine.

20
Table 2 Values of erf(y)

erf(y) y erf(y) y erf (y) y erf(y)

0.00 0.00000 0.25 0.27633 0.50 0.52050 0.75 0.71116

0.01 0.01128 0.26 0.28690 0.51 0.52924 0.76 0.71754

0.02 0.02256 0.27 0.29742 0.52 0.53790 0.77 0.72382

0.03 0.03384 0.28 0.30788 0.53 0.54646 0.78 0.73001

0.04 0.04511 0.29 0.31828 0.54 0.55494 0.79 0.73610

0.05 0.05637 0.30 0.32863 0.55 0.56332 0.80 0.74210

0.06 0.06762 0.31 0.33891 0.56 0.57162 0.81 0.74800

0.07 0.07886 0.32 0.34913 0.57 0.57982 0.82 0.75381

0.08 0.09008 0.33 0.35928 0.58 0.58792 0.83 0.75952

0.09 0.10128 0.34 0.36936 0.59 0.59594 0.84 0.76514

0.10 0.11246 0.35 0.37938 0.60 0.60386 0.85 0.77067

0.11 0.12362 0.36 0.38933 0.61 0.61168 0.86 0.77610

0.12 0.13476 0.37 0.39921 0.62 0.61941 0.87 0.78144

0.12 0.14587 0.38 0.40901 0.63 0.62705 0.88 0.78669

0.14 0.15695 0.39 0.41874 0.64 0.63459 0.89 0.79184

0.15 0.16800 0.40 0.42839 0.65 0.64203 0.90 0.79691

0.16 0.17901 0.41 0.43797 0.66 0.64938 0.91 0.80188

0.17 0.18999 0.42 0.44747 0.67 0.65663 0.92 0.80677

0.18 0.20094 0.43 0.45689 0.68 0.66378 0.93 0.81156

0.19 0.21184 0.44 0.45523 0.69 0.67084 0.94 0.81627

0.20 0.22270 0.45 0.47548 0.70 0.67780 0.95 0.82089

0.21 0.23352 0.46 0.48466 0.71 0.68467 0.96 0.82542

0.22 0.24430 0.47 0.49375 0.72 0.69143 0.97 0.82987

0.23 0.25502 0.48 0.50275 0.73 0.69810 0.98 0.83423

0.24 0.26570 0.49 0.51167 0.74 0.70468 0.99 0.83851

21
List of Figures

Figure 1 Simplified Drawing of An Experimental Probe

2 Construction Drawing of Probe Core Tube

3 Construction Drawing of Probe Handle

4 Construction Drawing of Probe Body

5 Construction Drawing of Protection Cap For Probe

6 Construction Drawing of Assembly Fixture Parts7 Assembly of Probe Parts

8 Sketch of Probe Assembly With Ribbon-Type Thermocouple

9 Schematic Diagram of Probe Wiring

10 Sketch of Probe Assembly With Fine-Wire Type Thermocouple

11 Electrical Schematic Diagram of Complete Instrument

12 Photograph of Printed Circuit Board, Component Side

13 Photograph of Printed Circuit Board, Opposite Side

14 Photograph of Component Mounted Board

15 Comparison of Measured and Calculated Values

16 Profiles of Various Skin Depth Temperatures

17 Effect of Thin Plastic Coating on Heated Brass

18 Physiological Response Curve

19 Reading Error Due to Variation In Thermal Inertia Value

20 Reading Error Due to Variation In Controlled Temperature At Probe

21 Photograph of Instrument and Probe

22
FRAME

SILICONE RUBBER

REFERENCE THERMOCOUPLE

JUNCTIONS

RESISTANCE THERMOMETER

HEATER WINDING

MEASURING THERMOCOUPLE

Fig. 1 Simplified Drawing of an Experimental Probe

23
I.43c« (.562")

.16 en

(.062")

SLIDE FIT IN PART C

2 HOLES

1.05am DRILL "HO DRILL

1 \Z\ l.47c« LJ_

1.562") r

CROSS SECTION AA

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2 ON EACH SIDE

1.05 nn DRILL *60 DRILL

&'

PAPER -BASE BAKELITE

TOLERANCE

(±O.OI5")±0.04cm

EXCEPT WHERE NOTED

Fig. 2 Construction Drawing of Probe Core Tube (Part A)

24
PART B

.32c»

(l/8

M )R

52 en HOLE 203")

.162 ei R

< 1/16") R

SNUG FIT IN PART C TOLERANCE

(±0.015") ±0.04cm

EXCEPT WHERE NOTED

CROSS SECTION A-A

NYLON

Fig. 3 Construction Drawing of Probe Handle (Part R)

25
CM

TO FIT PARTS A & B

TOLERANCE

(±0.015" )±0.04cm

EXCEPT WHERE NOTED

LUCITE

OR

NYLON

CROSS SECTION AA

Fig. 4 Construction Drawing of Probe Body (Part C)

26
SNUG FIT ON PART C

TOLERANCE

(+0.015" )±0.04cm

EXCEPT WHERE NOTED

3.17cm

Eo

(1.25")

2.54cn Ea

(1.000")

u 1.47cm i CM

CO

1

(.578") c3

'// Jr

t 7 /> 7 / / t 45

'///'///////////, ' f

CROSS SECTION A-A

NYLON

CM

Fig. 5 Construction Drawing of Protection Cap (Part D)
PART E *

^2t? 3

TOLERANCE

(±O.OI5")±0.04cm

EXCEPT WHERE NOTED

PART F

r U_2.54« I ]S

(1.25")

CROSS SECTION AA

ALUMINUM

N 3.5 X 0.6 TAP (6-32) TAP FOR THUMB SCREW

SLIDE FIT FOR PART C

TO FIT PLUG BELOW

0.6 an PITCH

( 40 THREADS PER INCH )

SLIDE FIT IN ABOVE PART

SCREWDRIVER SLOT

.081 en (.032") WIDE

.162 ea (.064") DEEP

BRASS

Fig. 6 Construction Drawing of Jig Assembly Fixture Parts

For Casting - Not Part of the Probe (parts E and F)

28
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PROBE CORE TUBE

RED LEAD (-)

ALUMEL

(MAG.)

HEATER WINDING

LAP JUNCTION

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BLUE LEAD(+)

JUNCTION OF TINNED COPPER WIRE LEADS

AND THERMOCOUPLE RIBBON

RESISTANCE THERMOCOUPLE ELEMENT

CHROMEL

RIBBON THERMOCOUPLE

Fig. 8 Sketch of Probe Assembly with Ribbon-Type Thermocouple

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JUNCTION OF TINNED —COPPER WIRE LEADS AND THERMOCOUPLE WIRE ELEMENTS

3MIL THERMOCOUPLE

WIRE ELEMENT

CHROMEL

SEE FIG. 8 FOR SIMILAR

DETAILS EXCEPT THERMOCOUPLE

Fig. 10 Sketch of Probe Assembly With Fine-Wire Thermocouple

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Fig. 12 Printed Circuit Board, Component Side

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Fig. 13 Printed Circuit Board, Opposite Side

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Fig. 14 Component Mounted Board

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Fig. 16 Profiles of Various Skin Depth Temperatures

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43
NBS-114A (rev. 7-73)

U S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC ATION OR REPORT NO. NBS TN-816

2. Gov't Accession

No.

3. Recipient's Accession Nc

4. TITLE AND SUBTITLE

ENGINEERING AND CONSTRUCTION MANUAL FOR AN INSTRUMENT

TO MAKE BURN HAZARD MEASUREMENTS IN CONSUMER PRODUCTS

5. Publication Date February 197U

6. Performing Organization (ode

7. AUTHOR(S) Louis A. Marzetta

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4460141, 4460503

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Consumer Product Safety Commission

Washington, D. C. 20016

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Surface temperature measurement alone is insufficient to establish the hazard of

human contact with a hot or cold object. A metal surface is more likely to cause

thermal injury than a plastic surface at the same temperature. An instrument equipped

with a measuring probe has been developed for indicating tbe tissue temperature that

would be experienced if human contact were made with the hot surface in question. The

correct value of interface contact temperature can be read for a selected contact time

without knowing the composition or temperature of the heated material under test.

A detailed set of instructions and drawings for assisting in the construction of

the device is included in the manual. Several test procedures are described for use

in checking the performance of the measuring probe and instrument.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Consumer products; contact temperature; thermesthesiometer ; thermometry.

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^2 For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

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